1. Field of the Invention
The present invention relates to a compact optical architecture for a wide-field helmet-mounted display, and can be applied more particularly to helmet-mounted display devices for aircraft or helicopter pilots.
A helmet-mounted display device enables the pilot to display flight information and/or images from sensors, for example of the light intensifier (LI) type. When coupled with a position detection system, it considerably increases the performance of the sighting system as compared with the head-up display which is fixed with respect to the axis of the aircraft.
The image presented to the pilot is projected either through a combiner (a semi-reflective piece of glass or plastic placed before the eye) or directly by means of the helmet visor. This approach has the advantage of providing a greater field of peripheral vision and therefore greater comfort of use. The helmet-mounted displays with visor projection that are currently found in the market can be used today to display fields in the range of 40xc2x0 (total field) on visors consisting of two spherical half-visors.
2. Description of the Prior Art
A need is now arising for a wider field (60xc2x0 to 80xc2x0) in order to give the pilot information in his peripheral field. This information makes it possible to carry out low-altitude, high-speed flights under improved conditions of security. The need to display wider fields in a helmet-mounted display rules out the use of a two-part visor. Indeed, beyond a horizontal field value in the region of +/xe2x88x9222xc2x0 (namely a 45xc2x0 field) and for a standard inter-pupil distance (63 mm), the separation between the two parts of the visor no longer provides for a total overlapping between the fields seen from the left eye and the right eye. The diagrams of FIGS. 1A and 1B thus illustrate the use of a two-part visor. FIG. 1A gives a very simplified top view of a two-part visor 11g (before the left eye OG) and 11d (before the right eye OD), with respective ocular axes xcex94g and xcex94d that are parallel, separated by a separation 12, in the case of a field xcex8 greater than 45xc2x0 (for example 60xc2x0). The ocular axis is defined by the axis that passes through the center of the pupil and is perpendicular to the plane of said pupil. FIG. 1B represents the fields seen from the left eye and the right eye in the case of a configuration of the type shown in FIG. 1A. It can be seen that the field 13g viewed from the left eye and the field 13d viewed from the right eye partially overlap in a region referenced 14 in FIG. 1B. For the pilot, this entails troublesome visual effects owing to increased luminance at the center. The separation 12 can be eliminated by implementing visor elements with divergent axes. However, this method further accentuates the inconvenience due to binocular overlapping. Furthermore, for an aircraft pilot""s helmet for example, the placing of a two-piece visor is not appropriate because it is a brittle structure that may break in the event of the pilot""s ejection.
FIG. 2 is a simplified and partial top view of a helmet-mounted display with a one-piece visor. As can be seen in this figure, the use of a one-piece visor (referenced 21 in FIG. 2) gives rise to difficulties of technical construction because it necessitates a relatively lengthy trajectory in the open between a first deflection mirror 22 and the image-forming means, referenced 24g on the path of the left eye OG (shown in a solid line in FIG. 2) and 24d on the path of the right eye OD (in dashes). This constraint arises out of the fact that the optical beam sent out by an image source (not shown in FIG. 2) is redirected in the case of a single-piece visor on the side opposite to the eye that it is supposed to illuminate. The vectors ng and nd represent the vectors normal to the surface of the visor 21 at the points of impact of the ocular axes xcex94g and xcex94d respectively of the left eye and the right eye. It is therefore necessary to fold the optical paths (especially by means of deflection mirrors referenced 23g et 23d in FIG. 2, respectively for the left eye and right eye channels) which give rise to optical paths that are lengthier than in the case of a two-piece visor. Now this propagation of light beams over a lengthy distance in the open leads to substantial beam diameters, necessitating large-sized image-formation means, that are incompatible with the compactness-related constraints of helmet-mounted displays.
The present invention proposes a new type of optical architecture for helmet-mounted display with visor projection, enabling the field of the visor to be augmented while at the same time meeting the constraints of mass and compactness dictated by the fact that the helmet is worn on the head.
More specifically, the invention proposes a compact optical architecture for a wide-field helmet-mounted display comprising at least one image source with a given field, a single-piece visor enabling the projection to infinity of said image before an eye of a pilot wearing the helmet, at least one optical relay to convey said image between the image source and the visor, the optical relay comprising image-formation means and at least two deflection mirrors, including one forehead-mounted mirror having a given surface area, the optical architecture being characterized in that:
said visor is formed by a surface generated by revolution, of which an axis of revolution is substantially parallel to the ocular axis and substantially contained in the sagittal plane of the pilot, at the level of the forehead,
the forehead-mounted mirror is not plane, having an axis of revolution that is the same as said axis of revolution of the visor, its geometrical parameters being computed in such a way that the envelope of the useful beams propagated between said forehead-mounted mirror and the first beam-formation means of the optical relay upstream from the forehead-mounted mirror are substantially contained in a cylinder with a section substantially equal to the useful surface of said mirror.